1. Field of the Invention
The present invention relates to a magnetic storage apparatus and a magnetic storage medium, particularly to a magnetic storage apparatus having a recording density of 50 Gb/in2 or higher and a magnetic storage medium to achieve the recording density.
2. Description of the Related Prior Art
In recent years, an amount of information handled by computers has been steadily increased, and a larger capacity and higher transfer rate are more and more required of a magnetic disk storage device of an external storage device. So far, a magnetic disk storage device having the maximum recording density of 10 Gb/in2 class has been commercialized. This kind of magnetic disk storage device adopts a longitudinal magnetic recording method. However, influence of a so called thermal fluctuation has become conspicuous, in a state where a magnetic energy possessed by becoming extremely fine recording bits decreases as the recording density increases and a recorded magnetization is reversed due to demagnetizing field working at a bit-transition and ambient heat. Therefore, it is considered to be difficult to attain an areal recording density exceeding 40 Gb/in2 in the conventional longitudinal magnetic recording method that uses a recording layer of a Co alloy series.
On the other hand, a perpendicular magnetic recording method is a magnetic recording method, in which magnetization is formed in a direction perpendicular to the surface of a recording medium film and such that adjacent recording bits become antiparallel to each other. Unlike the longitudinal magnetic recording method, the perpendicular magnetic recording method has a small demagnetizing field at the bit-transition, and has a characteristic that the magnetization is stably maintained as the recording density becomes higher. Accordingly, the perpendicular magnetic recording method is considered as one of the strong means for attaining a high recording density that exceeds the thermal fluctuation limit of the current longitudinal magnetic recording method. Media used in the perpendicular magnetic recording method are classified into two types; one is a single layer perpendicular magnetic recording medium having a perpendicular magnetic recording layer formed on a substrate via a non-magnetic underlayer, and the other is a double layer perpendicular magnetic recording medium where a soft magnetic underlayer is formed on a substrate and the perpendicular magnetic recording layer is formed on the soft magnetic underlayer directly or via a non-magnetic intermediate layer. In the single layer perpendicular magnetic recording medium, a ring-type head similar to the one used in the current longitudinal magnetic recording medium is generally used. However, since a gradient of a magnetizing field of the perpendicular magnetic recording is not steep, there is a problem that resolution is not improved. On the other hand, in the double layer perpendicular magnetic recording medium, a single pole type head can be utilized, where a strong magnetizing field of the perpendicular magnetic recording and a steep gradient of magnetizing field are obtained. As a result, it is advantageous that the resolution is improved in comparison with the single layer perpendicular magnetic recording medium. For this reason, a combination of the double layer perpendicular magnetic recording medium and the single pole type head is considered to be effective for commercializing the perpendicular magnetic recording method.
The double layer perpendicular magnetic recording medium can obtain the high resolution, but on the contrary, noise originated in the soft magnetic underlayer is problematic, in addition to noise originated in the recording layer, which can be seen in the single layer perpendicular magnetic recording medium as well. The noise is classified into a spike noise and a transition noise; the former occurs from a magnetic domain wall of the soft magnetic underlayer and the latter occurs by fluctuation of a magnetization transition in the recording layer owing to a magnetization state of the soft magnetic underlayer. With regard to the former spike noise, for example, as disclosed in Japanese Patent Laid-open No. 7(1995)-129946 gazette and Japanese Patent Laid-open No. 11(1999) 191217 gazette, there is a method where a hard-magnetic pinning layer is provided between the soft magnetic underlayer and the substrate to control a magnetic domain structure of the soft magnetic underlayer, thereby the spike noise is reduced. On the other hand, the latter transition noise is observed in a state of superposing the transition noise originated in the recording layer itself. Therefore, details are not yet clear as to how much the magnetization state of the soft magnetic underlayer influences the fluctuation of the magnetization transition in the recording layer.
When it is considered that the perpendicular magnetic recording method in the combination of the double layer perpendicular magnetic recording medium and the single pole type head is applied at the recording density exceeding the thermal fluctuation limit of the longitudinal magnetic recording method, both of a medium noise originated in the recording layer and the medium noise originated in the soft magnetic underlayer need to be reduced. The present invention has been created to solve the above-described problems. More specifically, the object of the present invention is to provide the perpendicular magnetic recording medium having a high medium S/N ratio at the recording density of 50 Gb/in2 or higher, and to facilitate the achievement of a high density magnetic storage device.